You can do this achievement in either offline practice mode, a private match with other friends or if you're lucky you can get it in a public match with someone who has two local players.
Assuming you don't have two friends who are willing to play this locally with you, it may be easier to have one person online with you and one of you two can have two local "dummies" playing against easy AI. Have one person playing a high damage character and another person playing the healer.
Personally, I think using these characters would be best: (this is assuming it's just two of you)
Healer/SupportVoltar - His Suicide Drones make for a great way to increase pressure as well as his Healbot and Techno Synaptic Wave make for great heals.
Deadlift - For the most part, this character is mostly here just for his Protective Pose to give a shield on himself and the other player. Past that, he's mostly just going to be another damage dealer with a helpful skill.
DamageRaelynn - While not having any real way to heal herself via skills, her "Snipe" skill alone makes this character almost able to one shot most NPC's early and mid game.
Clunk - High ranged damage as well as the "Vacuum Bite" giving the opportunity to salvage some health back if your healer/support is on CD.
CombinationsVoltar/Raelynn - In this instance, Voltar needs to be watching the map and setting the lanes to
slow push (different game, I know, but the method is the same. Kill one minion but not the other and let your minions slowly push while they build up in numbers) while Raelynn focuses on a single lane to push.
Deadlift/Clunk - So Deadlift offers some damage and more importantly, a shield. So with Clunk having a minor heal and already being tanky, a shield helps to further buff your Clunk into allowing more fight engagements.
Voltar/Deadlift - Could work, but in my opinion, requires far too much micromanagement and doesn't output enough damage.
I would like to again note, the options are only based on my personal opinion and for some, these characters may not work. I do hope that they do, and if anyone has some better character combinations for 2v4 scenario, please do voice them in the comments and I'll add them here. All in all, if you don't feel this solution is of any help, feel free to downvote. But before doing so, please do let me know in the comments what I could do better so that I can alter my post to be of more use. Enjoy and good luck!